This invention relates to filled thermoplastic polymeric resinous compositions in which the fillers have been modified to enhance their compatibility with polymeric resins and to improve the physical properties of products formed from the filled resin compositions.
Inorganic fillers or pigments, such as clays, titanium dioxide, barium sulfate, calcium sulfate, talc and the like are commonly blended with synthetic polymeric resins as extenders, and to provide opacity or color and/or to act as delustrants for articles formed from the resins. The quantity of such fillers used is usually very limited because larger amounts tend to reduce the strength of the article formed from the filled resinous composition to the point where they are unacceptable for their intended use.
Apparently, the decrease in physical integrity attendant with larger amounts of these inorganic fillers is related to the surface energy of both the filler and the polymeric resin. The surface of these inorganic fillers tend to be hydrophilic and organophobic. On the other hand, polymeric resins, particularly thermoplastic polyolefinic resins, tend to be hydrophobic. In other words, the resultant physical integrity is related to the adhesion of the polymeric resin to the inorganic filler. When there is good adhesion of the polymeric resin to the surface of the inorganic filler, i.e., the polymeric resin wets the surface of the inorganic filler, products formed from the filled resinous composition usually have increased physical strength. On the other hand, poor adhesion or wetting of the inorganic filler by the polymeric resin results in decreased physical strength, particularly the tear strength of thin films formed from filled thermoplastic polymeric resin compositions.
Various methods have been proposed to modify the surface energy of various inorganic fillers so as to make them readily dispersible or miscible in polymeric resins. An example of such a method is described in U.S. Pat. No. 3,697,474. In the method disclosed by this patent, reactive titanium esters of long chain olefinic acids are dissolved in an inert organic solvent, the surface of the inorganic filler particles, such as clay, is wetted with this solution and the titanium compound reacts with the inorganic filler to provide it with a hydrophobic, organophilic film. This grafting method provides good adhesion of the reactive portion of the ester molecule to the filler surface and produces a modified filler having a surface energy substantially matching that of various polymeric resin so that the modified filler is readily dispersible therein. This method has several practical disadvantages. For instance, expensive reagents and relatively large quantities of solvent are required. Also, the solvent and hydrolysis products must be removed from the modified filler by distillation or filtering. Furthermore, during extrusion of polymeric resins filled with such modified fillers, free acids contained in the filler tend to evaporate, causing an unpleasant odor in the surrounding area and in the product itself. This odor often remains with the product for several weeks, thereby limiting the commercial applications thereof.
Examples of other prior art methods for modifying inorganic fillers are disclosed in U.S. Pat. No. 3,004,858 wherein an alkylene oxide polymer is dissolved in a solvent and this solution is added to and stirred with a slurry of titanium dioxide pigment and U.S. Pat. No. 3,573,994 wherein a clay is first activated by heating to an elevated temperature of 300.degree.-700.degree. C and then polymerized with a divinyl ether monomer dissolved in a solvent. Both of these processes require the relatively expensive steps of removing the modified filler from the solvent media and drying to remove residual solvent.
Examples of still other prior art methods for modifying inorganic fillers are disclosed in U.S. Pat. Nos. 3,767,444, 3,642,510, 3,458,804 and 3,451,835.